Bond Chuck, Methods of Bonding, and Tool Including Bond Chuck

ABSTRACT

A bonding chuck is discussed with methods of using the bonding chuck and tools including the bonding chuck. A method includes loading a first wafer on first surface of a first bonding chuck, loading a second wafer on a second bonding chuck, and bonding the first wafer to the second wafer. The first surface is defined at least in part by a first portion of a first spherical surface and a second portion of a second spherical surface. The first spherical surface has a first radius, and the second spherical surface has a second radius. The first radius is less than the second radius.

BACKGROUND

The semiconductor industry has experienced rapid growth due tocontinuous improvements in the integration density of a variety ofelectronic components (e.g., transistors, diodes, resistors, capacitors,etc.). For the most part, this improvement in integration density hascome from repeated reductions in minimum feature size (e.g., shrinkingthe semiconductor process node towards the sub-20 nm node), which allowsmore components to be integrated into a given area. As the demand forminiaturization, higher speed and greater bandwidth, as well as forlower power consumption and latency, has grown recently, there has growna need for smaller and more creative techniques for formingsemiconductor structures.

As semiconductor technologies further advance, stacked semiconductordevices, e.g., 3D integrated circuits (3DIC), have emerged as aneffective alternative to further reduce the physical size of asemiconductor device. In a stacked semiconductor device, active circuitssuch as logic, memory, processor circuits, and the like are fabricatedon different semiconductor wafers. Two or more semiconductor wafers maybe installed on top of one another to further reduce the form factor ofthe semiconductor device. The stacked semiconductor devices may providea higher density with smaller form factors and allow for increasedperformance and lower power consumption. Further, by stackingsemiconductor devices, circuits can be formed on one semiconductor waferby processes that are incompatible with circuits formed and/or processesperformed on another semiconductor wafer, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross section of a multi-curvature bonding chuck inaccordance with some embodiments.

FIGS. 2A through 2C are various steps for wafer bonding using amulti-curvature bonding chuck in accordance with some embodiments.

FIG. 3 is an example bonding tool in accordance with some embodiments.

FIGS. 4A, 4B, and 5 through 9 illustrate components and operation of abonding module of a bonding tool in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” “top,” “bottom,” and the like, may be used herein forease of description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Some embodiments are discussed below in a specific context, namely waferbonding. Some specific examples are provided in which device and/orcarrier wafers are bonded. However, aspects of this disclosure may beapplied in numerous other contexts, such as to wafer bonding to achievea semiconductor-on-insulator (SOI) wafer, a strained semiconductorvirtual substrate, or the like. Further, some modifications to processesand systems are discussed below, and one of ordinary skill in the artwill readily understand additional modifications that can be applied.Embodiments contemplate these modifications. Further, although somemethods are described in a particular order, some embodimentscontemplate methods performed in any logical order.

FIG. 1 illustrates example geometric properties of a cross section of amulti-curvature bonding chuck in accordance with some embodiments. Someof these properties may be exaggerated and/or not proportional in FIG. 1and subsequent figures to clearly convey aspects of the illustratedembodiments. A surface of the bonding chuck is convex and includesportions of surfaces of two spheres having different radii. Insubsequent discussion, a portion of a surface of a sphere may bereferred to as a “spherical surface;” however, this alone does notnecessarily connote that an entire surface of a sphere is included. Afirst spherical surface S1 has a first radius R1 from a first center C1,and a second spherical surface S2 has a second radius R2 from a secondcenter C2. The first radius R1 is less than the second radius R2. Thefirst center C1 and the second center C2 align in a directionperpendicular to the planar chuck base 10, e.g., in a Z-direction. Thefirst center C1 is positioned relative to the second center C2 such thatthe first spherical surface S1 protrudes from the second sphericalsurface S2 in a Z-direction at a center line 20 of the sphericalsurfaces S1 and S2. The first spherical surface S1 is in a centerportion of the second spherical surface S2 on the bonding chuck. Inpractice, the bonding chuck may deviate from these spherical surfaces S1and S2 proximate a joint transitioning from the first spherical surfaceS1 to the second spherical surface S2 to allow for a smooth transition,for example. Other cross sections of the bonding chuck intersecting thecenter line 20 of the surfaces may be identical to the cross section ofFIG. 1.

Example dimensions of the bonding chuck, such as for bonding 300 mmwafers, are provided below. One of ordinary skill in the art willreadily understand that the dimensions can be varied, which iscontemplated within the scope of other embodiments. The first radius R1is about 90 m, and the second radius R2 is about 200 m. At a center ofthe surface of the multi-curvature bonding chuck, an apex height at thecenter line 20 from the planar chuck base 10 to the center of the firstspherical surface S1 is between about 100 μm and about 125 μm. Adistance 22 in an X-Y plane from the center line 20 to a junction of thefirst spherical surface S1 and the second spherical surface S2 is about60 mm. A distance 24 in an X-Y plane from the center line 20 to ajunction of the first spherical surface S1 and the planar chuck base 10is about 150 mm. At this junction of the first spherical surface S1 andthe planar chuck base 10, a distance 26 in a Z-direction to the secondspherical surface S2 is between about 20 μm and about 45 μm.

An appropriate vendor can manufacture and provide a bonding chuck havinga multi-curvature surface as described herein. For example, EV Group,headquartered in Austria, can provide such a bonding chuck.

FIGS. 2A through 2C illustrate various steps in wafer bonding using amulti-curvature bonding chuck. FIG. 2A illustrates a bottom bondingchuck 40 having a multi-curvature bonding surface 42, such as discussedin FIG. 1, and a top bonding chuck 44 with a bonding pin 46 in aretracted position. A first wafer 50 is on the multi-curvature bondingsurface 42 of the bottom bonding chuck 40, and a second wafer 52 is onthe top bonding chuck 44.

The first wafer 50 and the second wafer 52 can be any type of wafer. Forexample, each of the first wafer 50 and the second wafer 52 can be awafer comprising logic dies, system-on-chip (SOC) dies, applicationspecific integrated circuit (ASIC) dies, image sensor dies, memory dies,or the like. In an example, the first wafer 50 is a wafer comprising SOCdies, and the second wafer 52 is a wafer comprising ASIC dies. Inanother example, the first wafer 50 is a wafer comprising logic dies,and the second wafer 52 is a wafer comprising image sensor dies.

The first wafer 50 and the second wafer 52 can be prepared for bondingusing any acceptable process. Then, the first wafer 50 is placed andsecured on the multi-curvature bonding surface 42. A vacuum system maybe coupled to the bottom bonding chuck 40 and may be used to secure thefirst wafer 50. For example, many small holes or perforations may be inthe multi-curvature bonding surface 42 such that a pressure differentialmay be applied to a back (e.g., non-bonding) surface of the first wafer50. The first wafer 50 may be secured to the bottom bonding chuck 40such that the first wafer 50 conforms to the multi-curvature bondingsurface 42, which may be an elastic deformation. For example, when avacuum system is used, a suction or pressure differential applied to theback (e.g., non-bonding) surface of the first wafer 50 may cause thefirst wafer 50 to conform to the multi-curvature bonding surface 42.Similarly, the second wafer 52 is placed and secured on the top bondingchuck 44. A vacuum system may be coupled to the top bonding chuck 44 andmay be used to secure the second wafer 52. For example, many small holesor perforations may be in a surface of the top bonding chuck 44 to whichthe second wafer 52 will be secured such that a pressure differentialmay be applied to a back (e.g., non-bonding) surface of the second wafer52. In some embodiments, the vacuum system is turned on, andsubsequently, the second wafer 52 is placed on the top bonding chuck 44.With the vacuum system turned on, the second wafer 52 may be secured tothe top bonding chuck 44, for example, even if the surface of the topbonding chuck 44 to which the second wafer 52 is secured is facingdownwardly. The top bonding chuck 44 can be positioned above the bottombonding chuck 40 with the bonding surfaces of the first wafer 50 and thesecond wafer 52 facing each other.

In FIG. 2B, the bonding pin 46 extends from the top bonding chuck 44causing the second wafer 52 to deform and causing the bonding surfacesof the first wafer 50 and the second wafer 52 to contact. The firstwafer 50 and the second wafer 52 first contact while the first wafer 50is secured and conforms to the multi-curvature bonding surface 42.Further, the first wafer 50 and the second wafer 52 first contact whilethe second wafer 52 is secured to the top bonding chuck 44 and isdeformed by the extension of and force from the bonding pin 46.Accordingly, the first wafer 50 and the second wafer 52 are initiallycontacted while both the first wafer 50 and the second wafer 52 aredeformed, such as by an elastic deformation. The first wafer 50 and thesecond wafer 52 can be held in contact in this position for a period oftime, such as between about 5 seconds and 10 seconds. In this manner,the contacting portions of the first wafer 50 and the second wafer 52(e.g., generally the respective centers of the bonding surfaces of thefirst wafer 50 and the second wafer 52) may initiate bonding, such asthrough chemical reactions (e.g., to form covalent and/or ionic bonds)and/or atomic attractive forces (e.g., such as polar forces and/orhydrogen bonding). Once bonding is initiated, a bond wave may propagatebetween the bonding surfaces outwardly.

In FIG. 2C, the first wafer 50 is released from the bottom bonding chuck40, and the second wafer 52 is released from the top bonding chuck 44.For example, the suction or pressure differential caused by respectivevacuum systems may be removed. Once the first wafer 50 and the secondwafer 52 are released, each of the first wafer 50 and the second wafer52 may return to its natural form, such as a planar wafer. Additionally,the bond wave initiated, as discussed with respect to FIG. 2B, maycontinue to propagate outwardly causing reactions and/or atomicattractive forces to occur between remaining portions of the bondingsurfaces of the first wafer 50 and the second wafer 52. By contactingthe bonding surfaces in generally center areas and having a bond wavepropagate outwardly, voids and/or gas pockets can typically be avoidedat the bonding interface between the bonding surfaces of the first wafer50 and the second wafer 52.

FIG. 3 illustrates an example bonding tool in accordance with someembodiments. The bonding tool of FIG. 3 comprises a first load port 82,a second load port 84, a first module 86, a second module 88, a thirdmodule 90, a fourth module 92, a bonding module 94, a robot assembly 96,and a control module 98. Each of the first load port 82 and the secondload port 84 can be a front opening unified pod (FOUP) from which therobot assembly 96 can receive and/or to which the robot assembly 96 canload various wafers. The robot assembly 96 is configured to transferwafers among any of the first module 86, the second module 88, the thirdmodule 90, the fourth module 92, and the bonding module 94. The firstmodule 86, the second module 88, the third module 90, and the fourthmodule 92 may be any acceptable module, such as modules configured toprepare wafers for bonding. The control module 98 may be, e.g., aworkstation computer that is capable of implementing a recipe forcontrolling the operation of the bonding tool, including each module.The control module 98 may comprise one or more electronic controllersand/or processors that control an automated process of the bonding tool,such as in accordance with a recipe supplied by memory (e.g., anon-transitory medium) in the control module 98 or remote from thebonding tool.

FIGS. 4A, 4B, and 5 through 9 illustrate components and operation of thebonding module 94 in accordance with some embodiments. FIG. 4Aillustrates a view of a bottom bonding chuck 40 and a top bonding chuck44 from an X-direction. The bottom bonding chuck 40 has amulti-curvature bonding surface 42 as discussed in FIGS. 1 and 2Athrough 2C. Although not explicitly illustrated, the top bonding chuck44 has a bonding pin 46, as discussed in FIGS. 1 and 2A through 2C. Anoptical alignment measurement system 100 is disposed proximate thebottom bonding chuck 40 and the top bonding chuck 44, which will bediscussed in further detail in FIG. 4B. A first vacuum system 102 isdisposed on a backside of the bottom bonding chuck 40 (e.g., oppositefrom the side where the first wafer 50 will be loaded). A second vacuumsystem 104 is disposed on a backside of the top bonding chuck 44 (e.g.,opposite from the side where the second wafer 52 will be loaded). Thefirst vacuum system 102 and the second vacuum system 104 may comprisevarious hoses or the like to couple a vacuum pump to the respectivebottom bonding chuck 40 and the top bonding chuck 44. The hoses cancouple openings that extend to the surfaces of the respective bottombonding chuck 40 and the top bonding chuck 44 on which wafers will beloaded, such as the multi-curvature bonding surface 42 of the bottombonding chuck 40.

FIG. 4B illustrates additional features of the bottom bonding chuck 40,the top bonding chuck 44, and the optical alignment measurement system100. The optical alignment measurement system 100 includes microscopes106, 108, 110, and 112. Bottom microscopes 106 and 108 are disposed fromthe backside of the bottom bonding chuck 40. Top microscopes 110 and 112are disposed from the backside of the top bonding chuck 44. The opticalalignment measurement system 100 may be the SmartView alignment systemavailable from EV Group, headquartered in Austria.

In FIG. 5, a first wafer 50 is loaded on the bottom bonding chuck 40.The bottom bonding chuck 40 is first translated away from the topbonding chuck 44, e.g., in a Y-direction as illustrated, to ease loadingof the first wafer 50. The translation may be by tracks powered by amotor, by a screw shaft driven by a motor, or the like. The robotassembly 96 may then place the first wafer 50 on the upward facingsurface, e.g., the multi-curvature bonding surface 42, of the bottombonding chuck 40. The first vacuum system 102 is then turned on suchthat the first wafer 50 is secured to the bottom bonding chuck 40 andconforms to the multi-curvature bonding surface 42, as discussed withrespect to FIG. 2A. Prior to loading the first wafer 50 on the bottombonding chuck 40, the first wafer 50 may be transferred to and/or fromone or more of the first module 86, the second module 88, the thirdmodule 90, and the fourth module 92 in preparation for bonding. In FIG.6, the bottom bonding chuck 40, with the first wafer 50 thereon, istranslated, such as by tracks powered by a motor, by a screw shaftdriven by a motor, or the like, back to be in view of the opticalalignment measurement system 100.

In FIG. 7, a second wafer 52 is loaded on the top bonding chuck 44. Thetop bonding chuck 44 is first translated away from the bottom bondingchuck 40, e.g., in a Y-direction as illustrated, to ease loading of thesecond wafer 52. The translation may be by tracks powered by a motor, bya screw shaft driven by a motor, or the like. The second vacuum system104 is then turned on. The robot assembly 96 may then place the secondwafer 52 on the downward facing surface of the top bonding chuck 44. Thesecond vacuum system 104 secures the second wafer 52 to the top bondingchuck 44, as discussed with respect to FIG. 2A. Prior to loading thesecond wafer 52 on the top bonding chuck 44, the second wafer 52 may betransferred to and/or from one or more of the first module 86, thesecond module 88, the third module 90, and the fourth module 92 inpreparation for bonding. In FIG. 8, the top bonding chuck 44, with thesecond wafer 52 thereon, is translated, such as by tracks powered by amotor, by a screw shaft driven by a motor, or the like, back to be inview of the optical alignment measurement system 100.

The optical alignment measurement system 100 may then be used to alignthe first wafer 50 and the second wafer 52 for bonding. Various motors,such as stage motors, can drive each of the bottom bonding chuck 40 andthe top bonding chuck 44 in an X-direction and a Y-direction for finetuning of alignment of the first wafer 50 and the second wafer 52 forbonding. Further, other motors, such as software compensated spindlemotors, can drive the top bonding chuck 44 around a Z-axis to rotate thesecond wafer 52 into alignment with the first wafer 50 for bonding.

The alignment process may use the SmartView alignment system to alignthe first wafer 50 and the second wafer 52. First, the first wafer 50 onthe bottom bonding chuck 40 is positioned to be viewed by the opticalalignment measurement system 100, and the top bonding chuck 44 isretracted to not obscure the view of the optical alignment measurementsystem 100. The first wafer 50 on the bottom bonding chuck 40 is thenobserved by the top microscopes 110 and 112. The alignment mark on thefirst wafer 50 is found, and the image of the alignment mark isdigitized and is stored electronically. The first wafer 50 on the bottombonding chuck 40 is then retracted, allowing the second wafer 52 on thetop bonding chuck 44 to be brought into position. The second wafer 52 onthe top bonding chuck 44 is then positioned to be viewed by the opticalalignment measurement system 100. The second wafer 52 on the top bondingchuck 44 is then observed by the bottom microscopes 106 and 108 and isaligned to the existing digitized image of the alignment mark of thefirst wafer 50. The alignment may be performed by finely tunedtranslation of the top bonding chuck 44 in an X-direction and/or in aY-direction and/or by finely tuned rotation of the top bonding chuck 44around a Z-axis. The first wafer 50 on the bottom bonding chuck 40 withthe first wafer 50 thereon is then moved back to its measured position.The optical alignment measurement system 100 may be calibrated prior toalignment to aid proper alignment.

In FIG. 9, the first wafer 50 and the second wafer 52 are bonded. Thebonding process may occur as described in FIGS. 2B and 2C. For example,the bonding pin 46 extends from the top bonding chuck 44 causing thesecond wafer 52 to deform and causing the bonding surfaces of the firstwafer 50 and the second wafer 52 to contact. The first wafer 50 and thesecond wafer 52 first contact while the first wafer 50 is secured andconforms to the multi-curvature bonding surface 42 using the firstvacuum system 102. Further, the first wafer 50 and the second wafer 52first contact while the second wafer 52 is secured to the top bondingchuck 44 using the second vacuum system 104 and is deformed by theextension of and force from the bonding pin 46. Accordingly, the firstwafer 50 and the second wafer 52 are initially contacted while both thefirst wafer 50 and the second wafer 52 are deformed, such as by anelastic deformation. The first wafer 50 and the second wafer 52 can beheld in contact in this position for a period of time. The contactingportions of the first wafer 50 and the second wafer 52 may initiatebonding, such as through chemical reactions and/or atomic attractiveforces. Once bonding is initiated, a bond wave may propagate between thebonding surfaces outwardly.

Then, the first vacuum system 102 is turned off such that the firstwafer 50 is released from the bottom bonding chuck 40, and the secondvacuum system 104 is turned off such that the second wafer 52 isreleased from the top bonding chuck 44. Once the first wafer 50 and thesecond wafer 52 are released, each of the first wafer 50 and the secondwafer 52 may return to its natural form, such as a planar wafer.Additionally, the bond wave initiated may continue to propagateoutwardly causing reactions and/or atomic attractive forces to occurbetween remaining portions of the bonding surfaces of the first wafer 50and the second wafer 52. By contacting the bonding surfaces in generallycenter areas and having a bond wave propagate outwardly, voids and/orgas pockets can typically be avoided at the bonding interface betweenthe bonding surfaces of the first wafer 50 and the second wafer 52.

The bonded first wafer 50 and second wafer 52 may then be clampedtogether to secure the first wafer 50 and second wafer 52 for transport.The robot assembly 96 may remove the bonded first wafer 50 and secondwafer 52 from the bonding module 94 and transfer the bonded first wafer50 and second wafer 52 to one of the first load port 82 or the secondload port 84. The bonded first wafer 50 and second wafer 52 may then betransferred to another tool, such as to an annealing tool so that thebonded first wafer 50 and second wafer 52 may be annealed to increase abond strength of the first wafer 50 and second wafer 52, for example.

Some embodiments may achieve advantages. By deforming a bottom wafer,such as the first wafer 50 as described above, during the initialcontact during the bonding process, alignment of the bonded wafers maybe increased. The deformation of the bottom wafer can expand or enlargethe bonding surface of the bottom wafer to correspond more to theexpansion or enlargement of the top wafer when the bonding pin causesdeformation of the top wafer. Hence, features of the bonded wafers canalign more accurately, for example, to less than 0.3 μm. The improvedbonding accuracy can increase yield, which can in turn decrease anoverall cost of a product. Further, a multi-curvature bonding surfacecan be applied to any bonding tool to easily improve bonding accuracywithout further tool, materials, or processes being required.

An embodiment is a method. The method includes loading a first wafer onfirst surface of a first bonding chuck, loading a second wafer on asecond bonding chuck, and bonding the first wafer to the second wafer.The first surface is defined at least in part by a first portion of afirst spherical surface and a second portion of a second sphericalsurface. The first spherical surface has a first radius, and the secondspherical surface has a second radius. The first radius is less than thesecond radius.

Another embodiment is a method. The method includes securing a firstwafer to a multi-curvature bonding surface of a first bonding chuck. Thesecuring the first wafer causes the first wafer to deform. The methodfurther includes securing a second wafer to a second bonding chuck, andwhile the second wafer is secured to the second bonding chuck, deformingthe second wafer. While the second wafer is deformed and while the firstwafer is deformed by the securing to the multi-curvature bondingsurface, the second wafer is contacted with the first wafer. After thecontacting the second wafer with the first wafer, the first wafer isreleased from the multi-curvature bonding surface, and the second waferis released from the second bonding chuck.

A further embodiment is a tool. The tool includes a bonding module. Thebonding module includes a first wafer bonding and a second wafer bondingchuck. The first wafer bonding chuck has a multi-curvature surface onwhich a first wafer is to be loaded. The multi-curvature surface isdefined at least in part by a first portion of a first spherical surfaceand a second portion of a second spherical surface. The first sphericalsurface has a first radius, and the second spherical surface has asecond radius. The first radius is different from the second radius. Thesecond wafer bonding chuck has a second surface on which a second waferis to be loaded.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method comprising: loading a first wafer on first surface of afirst bonding chuck, the first surface being defined at least in part bya first portion of a first spherical surface and a second portion of asecond spherical surface, the first spherical surface having a firstradius, the second spherical surface having a second radius, the firstradius being less than the second radius; loading a second wafer on asecond bonding chuck; and bonding the first wafer to the second wafer.2. The method of claim 1, wherein the second portion of the secondspherical surface protrudes from the first portion of the firstspherical surface in a central region of the first surface.
 3. Themethod of claim 1, wherein the loading the first wafer on the firstsurface causes the first wafer to conform to the first surface.
 4. Themethod of claim 1, wherein the first bonding chuck comprises a vacuumsystem, the vacuum system securing and conforming the first wafer to thefirst surface when the first wafer is on the first surface.
 5. Themethod of claim 1, wherein the second bonding chuck comprises a bondingpin, the bonding the first wafer to the second wafer comprisingextending the bonding pin from the second bonding chuck.
 6. The methodof claim 1, wherein the second bonding chuck comprises a vacuum system,the vacuum system securing the second wafer to the second bonding chuckwhen the second wafer is on the second bonding chuck.
 7. The method ofclaim 1, wherein during the bonding the first wafer to the second wafer,each of the first wafer and the second wafer are deformed at least atinitial contact.
 8. The method of claim 1, wherein: the loading thefirst wafer comprises securing the first wafer to the first bondingchuck, the first wafer conforming to the first surface by the securing;the loading the second wafer comprises securing the second wafer to thesecond bonding chuck; and the bonding the first wafer to the secondwafer comprising: deforming the second wafer while the second wafer issecured to the second bonding chuck; contacting the second wafer to thefirst wafer while the second wafer is deformed and while the first waferconforms to the first surface by the securing the first wafer to thefirst bonding chuck; and after the contacting the second wafer to thefirst wafer, releasing the first wafer from the first bonding chuck andreleasing the second wafer from the second bonding chuck.
 9. A methodcomprising: securing a first wafer to a multi-curvature bonding surfaceof a first bonding chuck, the securing the first wafer causing the firstwafer to deform; securing a second wafer to a second bonding chuck;while the second wafer is secured to the second bonding chuck, deformingthe second wafer; while the second wafer is deformed and while the firstwafer is deformed by the securing to the multi-curvature bondingsurface, contacting the second wafer with the first wafer; and after thecontacting the second wafer with the first wafer, releasing the firstwafer from the multi-curvature bonding surface and releasing the secondwafer from the second bonding chuck.
 10. The method of claim 9, whereinthe securing the first wafer comprises using a first vacuum system, andthe securing the second wafer comprises using a second vacuum system.11. The method of claim 9, wherein the multi-curvature bonding surfacecomprises a convex surface.
 12. The method of claim 9, wherein themulti-curvature bonding surface comprises a first portion and a secondportion, the first portion being a first convex surface, the secondportion being a second convex surface, the first convex surface beingdistinct from the second convex surface.
 13. The method of claim 9,wherein the multi-curvature bonding surface is defined at least in partby a first portion of a first spherical surface and a second portion ofa second spherical surface, the first spherical surface being distinctfrom the second spherical surface.
 14. The method of claim 9, whereinthe multi-curvature bonding surface is defined at least in part by afirst portion of a first spherical surface and a second portion of asecond spherical surface, the first portion of the first sphericalsurface being in a central region of the multi-curvature bondingsurface, the second portion of the second spherical surfacecircumscribing the central region of the multi-curvature bondingsurface, a first radius of the first spherical surface being less than asecond radius of the second spherical surface. 15.-20. (canceled)
 21. Amethod comprising: securing a first wafer to a first chuck; securing asecond wafer to a second chuck; bonding a first center portion of thefirst wafer to a second center portion of the second wafer withoutcontacting first edge portions of the first wafer to second edgeportions of the second wafer while securing the first wafer to the firstchuck and securing the second wafer to the second chuck; and releasingthe first wafer from the first chuck and the second wafer from thesecond chuck to bond the first edge portions of the first wafer to thesecond edge portions of the second wafer using a bonding wavepropagating from the first center portion of the first wafer and thesecond center portion of the second wafer to the first edge portions ofthe first wafer and the second edge portions of the second wafer. 22.The method of claim 21, wherein the first chuck comprises amulti-curvature bonding surface, and wherein securing the first wafer tothe first chuck comprises conforming the first wafer to themulti-curvature bonding surface using a vacuum system.
 23. The method ofclaim 22, wherein the multi-curvature bonding surface comprises a firstportion of a first spherical surface and a second portion of a secondspherical surface, wherein the first portion of the first sphericalsurface is disposed in a center region of the multi-curvature bondingsurface, wherein the second portion of the second spherical surfacecircumscribes the center region of the multi-curvature bonding surface,and wherein a first radius of the first spherical surface is less than asecond radius of the second spherical surface.
 24. The method of claim21, wherein the second chuck comprises a bonding pin, and bonding thefirst center portion of the first wafer to the second center portion ofthe second wafer comprises deforming the second wafer by extending thebonding pin onto a back surface of the second wafer opposite a bondingsurface of the second wafer contacting the first wafer.
 25. The methodof claim 24, wherein securing the second wafer to the second chuckcomprises using a vacuum system to suction the back surface of thesecond wafer.
 26. The method of claim 21, further comprising aligningthe first wafer and the second wafer using an optical alignmentmeasurement system.